Oak Preservation and Parking Yield on a Sloped Site
A 6.2-acre suburban site is being evaluated for a 120-space parking lot and an 18,000-square-foot community building. A topographic survey shows slopes ranging from 8% to 22%, with a broad flat shelf near the center. The soils map indicates silt loam with a measured infiltration rate of 0.15 inch per hour; bedrock was encountered at 4 to 6 feet in two of five borings. A 42-inch-diameter white oak stands near the center of the flat shelf, and a jurisdictional stream lies 80 feet from the proposed parking limit. The survey was flown in leaf-off conditions, and no wetland delineation or geotechnical report has been provided. The client wants a 'low-impact' layout and asks whether the oak can be saved while keeping all 120 spaces.
1. Identify the three most important data gaps and explain how each could change the layout.
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The three critical gaps are wetland jurisdiction, geotechnical conditions, and the oak's root zone.
- Wetland/stream delineation: the stream is 80 ft from the parking limit, but the ordinary high-water mark and jurisdictional buffer are unknown. A 50-ft or 100-ft buffer could eliminate parking rows or require a relocated stormwater outlet.
- Geotechnical report: bedrock at 4–6 ft in two borings affects infiltration, stormwater practices, and foundation depth. The measured 0.15 in/hr infiltration rate is marginal for a bioretention cell and may require an underdrain or a larger footprint.
- Arborist/tree survey: the 42-in oak's critical root zone is roughly 42–63 ft in radius, depending on the standard used. Without accurate root mapping and grade-change limits, any layout near the flat shelf is provisional.
These are not checklist items; each can shift the building envelope, parking count, and stormwater strategy.
2. The client asks whether the oak can be saved with all 120 spaces. What analysis would you perform, and what is your preliminary judgment?
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Test the oak against the parking program before promising 120 spaces.
- Calculate the protected area: at 1 ft radius per inch DBH, the CRZ is about 42 ft radius (5,540 sf); at 1.5 ft per inch, about 63 ft (12,470 sf).
- A 120-space lot with aisles typically needs 40,000–50,000 sf, plus circulation and stormwater. The flat shelf may be large enough, but the oak sits near its center, so a conventional double-loaded aisle layout will likely encroach on the CRZ.
- Preliminary judgment: saving the oak and all 120 spaces is possible only with a clustered layout, angled or compact stalls, an overflow lot, or a reduced space count. Do not promise it before an arborist maps roots and the civil engineer confirms grade changes.
- If fill or cut exceeds 6–12 in within the CRZ, preservation becomes a construction-protection problem, not just a layout problem.
3. What field verification would you require before finalizing the inventory and analysis?
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Require field verification of the survey and site conditions.
- Wetland flags and OHWM by a qualified delineator; confirm the stream's jurisdiction and buffer.
- Soil borings and infiltration tests at the actual stormwater location, not just the mapped soil unit.
- Tree locations, DBH, crown spread, root flare, and deadwood; leaf-off survey may have missed understory and evergreen obstruction.
- Spot elevations at the flat shelf, stream bank, and proposed parking limit; verify contours.
- Utility locate, including the culvert and any field tiles.
Without these, the analysis is a desk study. The exam expects you to know when a constraint is unverified and to recommend the specific field work rather than design on assumptions.